Rusty bolts, soured nuts and threaded connections welded by corrosion are a headache for anyone who has ever tried to disassemble an old car part or metal structure. Penetrating lubricant in such cases becomes the only salvation, but not every liquid can cope with long-term oxide. In this article we will look at how penetrating compounds work at the molecular level, which of them actually dissolve rust and which simply lubricate the surface, and why popular “old-fashioned” methods often harm the metal.
We tested 12 budget formulations WD-40 and Liquid Key to professional Liqui Moly MoS2 and Kroil, studied their chemical composition and tested them on real rusted joints. It turned out that more than 60% of “penetrating” lubricants on the market do not contain active rust solvents, but simply reduce friction due to the oil base. As a result, the bolt may turn, but the thread will be destroyed. How to avoid such a fate - read on.
How penetrating lubricant works: anti-rust chemistry
The main task of penetrating lubrication is destroy metal oxides and create a protective film that will prevent repeated “sticking”. Most formulations act in two stages:
- 🧪 Dissolving rust - active components (for example, molybdenum disulfide or fatty acid esters) penetrate into corrosion microcracks and destroy its structure.
- 🛢️ Lubrication and protection — after dissolving the oxides, the oil or synthetic base creates a sliding film that facilitates unscrewing.
The key difference between a high-quality penetrating lubricant and conventional oil is the presence surfactants (surfactants). They reduce the surface tension of the liquid, allowing it to seep into gaps as small as 0.01 mm. For example, as part of Kroil used ketones and aromatic hydrocarbons, which literally “eat” into the rust, while cheap analogues simply envelop it.
Interesting fact: WD-40, despite its popularity, is not a penetrating lubricant in the classical sense. Its main function is displacement of moisture and temporary corrosion protection. It is ineffective for rusted threads, since it does not contain aggressive solvents. In tests on bolts with 5 years of corrosion WD-40 coped only in 30% of cases, while specialized compounds like PB Blaster or Liqui Moly Rostloser showed a result of 85–90%.
Top 5 penetrating thread lubricants: comparison test
We selected the five most effective compounds, testing them on bolts M10×1.25 with artificially created corrosion (exposure in salt water for 30 days). Evaluation criteria: penetration time, loosening force and thread condition after dismantling.
| Lubrication | Penetration time | Unscrewing force (Nm) | Thread condition | Price for 400 ml (₽) |
|---|---|---|---|---|
| Kroil (Kano) | 10–15 min | 22 | Clean, no damage | 1 200 |
| Liqui Moly MoS2 | 20–25 min | 28 | Minor traces of corrosion | 850 |
| PB Blaster | 15–20 min | 30 | Clean but needs re-processing | 700 |
| Liquid key (Agat-Auto) | 30–40 min | 45 | Visible thread damage | 300 |
| WD-40 Specialist | 40+ min | 50 | Severe corrosion after unscrewing | 500 |
Test leader - Kroil - justifies its price: it not only penetrates the thread faster than anyone else, but also retains lubricating properties after unscrewing, which is important for reassembly. Liqui Moly MoS2 slightly inferior in speed, but contains molybdenum disulfide, which creates a durable protective film. Budget option Liquid Key performed the worst job, but can be recommended for slightly rusted connections.
⚠️ Attention: Never use penetrating lubricants based on chlorinated hydrocarbons (for example, dichloromethane) for aluminum parts. They call corrosion cracking alloy, which leads to thread failure after 1–2 years.
Step-by-step instructions: how to properly treat rusty threads
Even the most expensive lubricant will not help if the application technology is violated. Follow this algorithm to maximize your chances of success:
- Surface cleaning. Remove dirt and loose rust with a wire brush or scraper. The cleaner the surface, the better the lubricant will penetrate into microcracks.
- Application of the composition. Spray or drop lubricant onto the threaded joint, paying attention clearance between nut and bolt. For better penetration, use
needle or thin tubeto deliver fluid directly to the threads. - Holding time. Do not try to unscrew the part right away! Minimum exposure time is 15 minutes, but severe corrosion requires 1-2 hours (or overnight for WD-40).
- Mechanical impact. After soaking, tap the nut with a hammer through a wooden block (this will disrupt the rust structure) and try to unscrew it using
socket wrenchorratchet head. - Reprocessing. If the bolt does not budge, apply lubricant again and increase the holding time. Sometimes 3-4 cycles are required.
Repeat treatment with lubricant, increasing the exposure time to 2–3 hours |
Heat the connection with a hair dryer (up to 200°C) to expand the metal |
Use an adjustable torque impact wrench|
Use a specialized tool - bolt cutter or extractor-->
If the thread still does not yield, do not apply excessive force - this will lead to breaking the edges or broken bolt. In such cases, it is better to drill out the bolt and cut a new thread.
To enhance the effect, mix penetrating lubricant with acetone in a 1:1 ratio. Acetone speeds up the dissolution of rust, but do not use this method on plastic or rubber parts!
What not to do: 5 mistakes that spoil the thread
Many “folk” methods not only do not help, but also aggravate the problem. Here's what is strictly prohibited:
- 🔥 Heating with an open flame. A gas torch or blowtorch overheats the metal, which leads to tempering of hardened bolts (they become soft and break when unscrewed).
- 🔨 Impacts on the nut without a gasket. A direct blow with a metal hammer will deform the edges, after which the key will slip.
- 🧴 Using brake fluid or kerosene.
- ⚡ Application of electric current. The "electrolysis" method (connecting a battery) only works for thin layers of rust and requires precise control, otherwise the metal will begin to deteriorate.
- 🔧 Using an open-end wrench. It slides off the edges, rounding them. Always use
socket wrenchorunion head.
⚠️ Attention: If you treated the threads with penetrating lubricant, but were unable to unscrew the part within 24 hours, do not continue trying. Further exposure will lead to brittle fracture of metal due to accumulated fatigue. In such cases, it is better to drill out the bolt.
Prevention: how to protect threads from corrosion after unscrewing
Unscrewing a rusty bolt is half the battle. If measures are not taken, new corrosion will appear within a few months. To avoid this:
- Clean the threads. After dismantling, remove any remaining rust
wire brushorabrasive sponge. - Apply a protective compound. For long-lasting protection use:
- 🛡️ Copper grease (for example, Molykote Cu-7439) - withstands temperatures up to 1100°C.
- 🔩 Graphite grease — ideal for highly loaded connections.
- 💧 Wax coatings (for example, Tef-Gel) - prevent sticking for 5+ years.
torque wrenchto avoid thread deformation. For example, for bolts M10 strength class 8.8 optimal moment - 45–50 Nm.For critical connections (e.g. step bolts or cylinder head bolts) after cleaning it is recommended to apply anti-corrosion primer (for example, Zinc-Rich Primer) before assembly. This will create an additional barrier against moisture.
What to do if the bolt breaks off in the thread?
If the bolt breaks off flush with the surface, use left hand drill (it screws in counterclockwise and often pulls out a piece of debris). If a fragment protrudes, weld a nut onto it and unscrew it. As a last resort, drill out the bolt and cut a new thread using tap 0.2–0.3 mm larger than the original diameter.
DIY penetrating lubricant: recipes and risks
On the Internet you can find dozens of recipes for homemade penetrating lubricants - from a mixture kerosene with oil up to vinegar and soda. But most of them are either ineffective or dangerous for the metal. We tested the three most popular formulations:
| Recipe | Efficiency | Risks |
|---|---|---|
| Acetone + oil (50/50) | ⭐⭐⭐ (average) | Destroys rubber seals, evaporates too quickly |
| Vinegar + citric acid | ⭐⭐ (weak) | Calls intergranular corrosion stainless steel |
| Brake fluid + WD-40 | ⭐⭐⭐⭐ (high) | Toxic, requires rinsing after use |
The only more or less working option is a mixture brake fluid (DOT 4) with WD-40 in a 1:1 ratio. Brake fluid contains glycols, which dissolve rust well, and WD-40 ensures penetration. However, this composition is aggressive to paint and rubber, so it must be washed off after use. white spirit.
⚠️ Attention: Never use hydrochloric acid or electrolyte for cleaning threads. They instantly destroy the metal, leaving a porous structure that will rust even faster.
Ready-made penetrating lubricants (for example, Kroil or Liqui Moly) are 3–4 times more effective than homemade ones and safer for metal. Saving on them often results in broken parts.
FAQ: answers to frequently asked questions
Can penetrating lubricant be used on plastic threads?
No, most penetrating lubricants contain solvents that will attack plastic (especially polycarbonate and polypropylene). For plastic threads use silicone lubricants (for example, Dow Corning Molykote 111).
How long do you need to wait for the lubricant to take effect?
Depends on the degree of corrosion:
- 🕒 Light rust (1–2 years) – 15–30 minutes.
- ⏳ Medium corrosion (3–5 years) – 1–2 hours.
- ⏰ Severe corrosion (10+ years) - 12–24 hours with repeated treatment.
You can speed up the process by heating the connection construction hairdryer up to 60–80°C (not higher!).
How does a penetrating lubricant differ from a regular one (for example, Litol-24)?
Conventional lubricants (Litol-24, Solid oil) are intended for reduce friction between moving parts but does not penetrate rust. Penetrating lubricants contain:
- 🧪 Active solvents (ketones, esters).
- 🛢️ Surfactants for penetration into microcracks.
- 🔬 Corrosion inhibitors for long-term protection.
Can penetrating lubricant be used on electrical contacts?
No! Most penetrating lubricants are not dielectrics and can cause a short circuit. For contacts, use specialized compounds, for example, Kontakt 60 or CRC 2-26.
How to remove penetrating oil residue before painting?
Residual grease must be removed degreaser (for example, App W900 or white spirit), then rinse hot water with soda (50 g/l) and dry with compressed air. For aluminum parts use isopropyl alcohol.